PHOTO UNIT 17: THE MONOCOT STEM

OBJECTIVES:

Identify characteristics of monocot stems as distinguished from dicot stems.

State the method by which secondary growth is accomplished in monocots.

Identify primary thickening meristem in Yucca as seen in stem tip photos.

Describe the vascular pattern and bundle composition in primary stems.

Differentiate between primary and secondary bundles in Dracaena and describe the origin of each.

Photos for this study:

17-1: Acorus (sweet flag); Transverse section of rhizome

17-2: Triticum (wheat)' T.s. Eustele LP

17-3: Yucca (Spanish bayonet); Longitudinal section of stem tip

17-4: Yucca (Spanish bayonet); L.s lower down the stem apex

17-5: Dracaena (dracaena); T.s. stem primary bundle

17-6: Dracaena (dracaena); T.s. stem primary and secondary

17-7: Dracaena (dracaena); T.s. stem, early secondary bundle, HP

17-8: Dracaena (dracaena); T.s. stem, later secondary bundle HP

17-9: Dracaena (dracaena); T.s. secondary bundle HP

PHOTO STUDY 17-1: Acorus (sweet flag); Transverse section of rhizome.

Vascular bundles are disposed without apparent order in Acorus, just as you found them to be in Scirpus. They constitute a Meristele, the type of stele you have come to expect in monocotyledon stems. But note that, in this meristele, an endodermis is differentiated, marking the boundary between cortex and stele. Identify the very obvious Casparian strips in cells of the endodermis. Vascular bundles in the cortex are probably all leaf traces. Acorus, like Scirpus, is a plant living in marshy situations, which fact accounts for the numerous and large intercellular spaces in the ground tissue, two of which can be seen at the left margin of this image. Vascular bundles, are mostly amphivasal.

Despite the fact that wheat is a monocotyledon, the vascular bundles of its stem are distributed in a circular fashion, much like those of a dicotyledon. See, however, that there are two circular sets of bundles, the outer set smaller than the inner one and embedded in a layer of sclerenchyma tissue. Note too that fibrous sclerenchyma tissue extends radially in a band from each outer bundle to the epidermis. Alternating with these bands are the chlorenchyma tissue groups of the cortex. Note that all bundles are collateral, and that cambium appears in none of them. Those in the outer circle are leaf traces, supplying the next leaf above the level of this section. Instead of a central pith there is, in a mature internode, a large pith cavity.

Follow the section back from the apex. Beginning at about the level of the third or fourth leaf primordium, and extending from there back, see evidence of pronounced meristematic activity near the stem surface (11 o'clock). In a band of tissue that thickens perceptibly as you progress farther and farther back from the apex, find the cells to be regularly aligned in anticlinal rows.

Repeated periclinal cell division have greatly increased the diameter of this young stem before there has been any cessation in longitudinal growth; for you see that internodes are still extremely short, and leaves are consequently crowded. Such growth is primary, despite the fact that its result is an increase in stem diameter. In appearance, this is somewhat like cambial activity; but see the difference between derivatives of this meristem and those of a cambium, where one kind of tissue is added to one side and a second kind to the other side. Notice that procambium strands, as well as ground tissue, differentiate here. Such a meristem is not uncommon in monocotyledon stems. It brings about diameter growth in the absence of a vascular cambium. It is called a primary thickening meristem.

PHOTO STUDY 17-5: Dracaena (dracaena); T.s. of a stem primary bundle.

The cambium in Dracaena is quite different from the stelar cambium of a dicotyledon. Here it arises in the inner cortex, rather than in procambium; and instead of adding xylem tissue to one side and phloem tissue to the other, it adds secondary parenchyma only toward the primary cortex, and both secondary parenchyma and whole vascular bundles toward the stem center. Such bundles are, of course, wholly secondary. Relatively few monocotyledons exhibit secondary growth; but when they do, it is of this type, and not the type occurring in most dicotyledons.

This plant develops a meristele. Primary collateral vascular bundles are irregularly distributed throughout the pith. Identify protoxylem, metaxylem, phloem, and bundle sheath of a single primary bundle.

Identify the secondary amphivasal vascular bundle with xylem beginning to surround a small cluster of phloem cells. Xylem will continue to differentiate from cambial activity to complete the amphivasal identity of the bundle.

PHOTO STUDY 17-9: Dracaena (dracaena): T.s. secondary bundle HP

Deeper in the zone, xylem cells may differentiate around the phloem strand, and finally the bundle sheath of sclerenchyma tissue matures.

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PHOTO STUDY 17-1: Acorus (sweet flag); Transverse section of rhizome.

Vascular bundles are disposed without apparent order here, just as you found them to be in Scirpus. They constitute a meristele, the type of stele you have come to expect in monocotyledon stems. But note that, in this stem, an endodermis is differentiated, marking the boundary between cortex and stele. Identify the very obvious Casparian strips in cells of the endodermis. Vascular bundles in the cortex are probably all leaf traces. Acorus, like Scirpus, is a plant living in marshy situations, which fact accounts for the numerous and large intercellular spaces in the ground tissue. Vascular bundles, as you noted in an earlier observation, are mostly amphivasal.